1. Field of the Invention
The present invention relates, in general, to disk data storage devices, and, more particularly, to software, systems and methods for enhanced dibit extraction to compensate for non-linearity in correction circuits of hard disk drives.
2. Relevant Background
Hard disk drives provide low cost, reliable, and large capacity data storage for computing devices ranging from familiar personal computers to network storage used in data centers. Increasingly, hard disk drives are used for data storage in a variety of appliances such as printers, televisions, set top boxes, media centers, and portable media/data storage devices. Accordingly, there is a constant need to improve performance, reliability, and capacity for hard disk storage devices.
Disk drives typically include one or more rotating platters having a magnetic surface encased within an environmentally controlled housing. One or more read/write heads read and write data to the magnetic surface. Servo mechanisms precisely control placement of the read/write heads over locations on the magnetic surface and control circuits drive the servo mechanisms to control head positioning. The read/write heads are driven by read channel circuitry which operates to generate and/or sense electromagnetic fields on the platters. The electronics encode data received from a computing device and translate the data into magnetic encodings, which are written onto the magnetic surface by the read channel circuitry. When data is requested, the servo mechanisms locate the data, sense the magnetic encodings, and translate the encodings into binary digital information.
The read/write heads detect and record the encoded data as areas of magnetic flux. The data are encoded by the presence or absence of a flux reversal between two contiguous locations of the platter. Data may be read using a method known as “peak detection” by which a voltage peak imparted in the read/write head is detected when a flux reversal passes the read/write head. Increasingly, however, a technique called partial response maximum likelihood (“PRML”) is used to read/write data to the disk surface. PRML involves digitally sampling an analog “playback voltage” to determine a most-likely bit pattern represented by the analog waveform. PRML technology tolerates more noise in the magnetic signals, permitting greater manufacturing tolerances in the platters and read/write heads, which increases manufacturing yields and lowers costs.
Read channel electronics include correction circuits to compensate for these nonlinearities. In the absence of nonlinear effects, the playback voltage in a hard disk drive can be described by:
      V    ⁡          (      t      )        =      ∑                  1        2            ⁢              (                              a                          k              +              1                                -                      a            k                          )            ⁢              h        ⁡                  (                      t            -                          k              ⁢                                                          ⁢              T                                )                    where akε(1,−1) represents the NRZ (Non-Return to Zero) write current, T is the bit period and h(t) is the playback voltage response to an isolated transition response. In a hard disk drive, the magnetic read/write channel induces a variety of nonlinearities that usually require some form of compensation or correction. The more accurately these nonlinearities can be characterized, the more accurately they can be compensated for, which, in turn, allows the read channel to be calibrated for improved performance.
A paper containing useful background information concerning such distortions has been authored by Palmer, D and Ziperovich, P., and is titled “Identification Of Nonlinear Write Effects Using Pseudorandom Sequences”, IEEE Transactions On Magnetics, Vol. Mag-23, No. 5, September 1987, pp 2377–2379. This paper discusses linear and nonlinear distortions that occur in read channels, and describes a technique for separating linear and nonlinear effects, based on the unique properties of a maximal-length pseudorandom noise sequence (a “PN sequence”).
One technique described in this paper is referred to as “dibit extraction”. Dibit extraction can be used to calibrate the nonlinear correction circuits used to compensate for these nonlinearities. A wide variety of measurements can also be obtained from an extracted dibit. A big advantage of dibit extraction is that it can simultaneously detect a number of nonlinear effects during the testing and calibration phase of manufacturing. Dibit extraction can also be used to estimate or minimize these nonlinear effects. In this manner dibit extraction is a technique that can perform a multitude of functions that can reduce manufacturing time and cost. However, these functions depend on extracting accurate dibit response information.
Dibit extraction involves extracting the dibit response from the playback voltage V(t). In a typical application, a maximal length pseudorandom noise (PN) sequence (also referred to as an “m-sequence”) is written to disk (e.g., a 127 bit PN sequence) and then read back to obtain the playback voltage V(t). When ak is a maximal length NRZ pseudorandom sequence, then nonlinear effects are associated with various products of NRZ values, e.g., magnetoresistive (MR) asymmetry is associated with akak−1, and NLTS (non-linear transition shift) is associated with ak−1akak+1. Based on the “shift and add property” of maximal length pseudorandom sequences, the nonlinear NRZ products produce echoes in the extracted dibit.
Some nonlinear products and echo locations in the extracted dibit are shown in prior art FIG. 1, and are shown in table 1 below for a length 127 maximal length pseudorandom sequence (or m-sequence) defined by:xn=xn−3xn−7 where x0= . . . =x6=−1.
TABLE 1(Prior art)Location Relative to MainDibit Response forAssociated Nonlinearxn = xn−3xn−7 withNonlinearityTermx0 = • • • = x6 = −1NLTS, PEak−1akak+1 −25.5, −1.5NLTS2ak−2akak+1 +15HTakak+1+30.5MRakak−1, ak−2, akak−3+31.5, +61, +5.5MR Sat.ak−1akak+1,−25.5, +13.5, −44.5ak−2akak+1,ak−1akak+2KEY:NLTS = Nonlinear Transition ShiftNLTS2 = NLTS from 2 Bits Periods awayPE = Partial ErasureHT = Hard TransitionMR = Magneto-Resistance
When the m-sequence, ak, is deconvolved with V(t), the result is the dibit response:d(t)=h(t)−h(t−T)which is the response of the channel to an isolated NRZ pulse (also called a dibit because it consists of two transitions one bit width apart). Deconvolution is a process of finding the most likely input to a system, given the known properties and measured outputs of the system. In most cases, a sampled dibit response:dk=d(kT)=h(kT)−h((k−1)T)is used. In the case of longitudinal recording, the main dibit response has the shape of a dipulse, as shown in FIG. 1, whereas in the case of perpendicular recording, the dibit response has the shape of an isolated pulse (not shown).
An example of a longitudinal dibit response 101 is shown in prior art FIG. 1 with some of the possible nonlinear echoes (e.g., echoes 103, 104 and 106). In FIG. 1, the vertical axis represents the amplitude of the dibit response signal in volts and the horizontal axis represents the discrete time shifts of the PN sequence. In FIG. 1, there are 127 values spread across the horizontal axis in a range of discrete time shifts (measured in bits) from between −63 bits to +63 bits. The main dibit response 101 represents the linear response of the read channel. The location of each echo is a result of “shift and add” property of the m-sequence interacting with a particular nonlinearity, hence, the location of each echo can generally be predicted. The height and polarity of each echo is related to the type of nonlinearity associated with the echo. For example, at moderate to high channel densities, the NLTS echo height 103 relative to the main dibit 101 is approximately one half the actual NLTS percentage. The shape of the echo may also indicate the origin of the echo. For example, the HT echo 104 at 30.5 is asymmetric whereas the MR echo 106 at 31.5 is symmetric.
An accurate dibit response can be used to characterize nonlinearities in the read channel and compensate for those nonlinearities. For example, write pre-compensation can be used to adjust the signal as it is written to disk so that the nonlinearities have less affect on the read data. Accurate dibit response data can be used to estimate channel pulse width parameters such as PW50 (average pulse width at 50% of peak level in a signal) and T25–75 (the transition width from 25% peak signal to 75% peak signal). Also, dibit information can potentially be used to estimate the overwrite ratio.
One problem with current dibit extraction techniques is that sometimes the nonlinear echoes interfere with each other or with the main dibit response. This occurs when a nonlinear echo occurs coincidently with (e.g., at substantially the same location as) the main dibit response or another echo to produce constructive or destructive interference. This situation makes dibit extraction based calibration and measurement less useful or inaccurate. Hence, a need exists for systems and methods for dibit extraction that overcome the limitations caused by echoes that interfere with accurate characterization of the read/write channel.